With the aim of achieving accurate and efficient transmission of high-speed high frequency signals, heretofore it has been customary to adopt a differential transmission line structure as an internal line structure of a packaging component for mounting and sealing a fast-operating semiconductor device such as IC and LSI or a surface-mount type electronic component. In a differential transmission line structure of conventional design, a pair of signal conductors are arranged side by side on a main surface of a dielectric substrate serving as a support and as an insulator as well.
In such a differential transmission line structure, a pair of signals in phase opposition are transmitted through a pair of signal conductors. This makes it possible to decrease an amplitude of each signal and thereby achieve the speeding up of data transmission and also cancellation of external noise.
Conventional differential transmission line structures are typically designed in the form of a parallel microstrip line, and more specifically a structure in which signal conductors are arranged side by side on a main surface of a dielectric substrate, or the form of a parallel strip line in which such a parallel arrangement of signal conductors assumes an internal layer pattern. In such constructions, however, since the signal conductors are opposed to each other at their side faces, it follows that its opposed area is so small that the electrical coupling established between the signal conductors tends to be not strong enough.
In order to achieve fine-pitch arrangement of electrode terminals in a semiconductor device, for example, in a multilayer circuit board composed of a plurality of dielectric layers, it is desirable to adopt a differential transmission line structure composed of a pair of signal conductors arranged face to face with each other in a direction of substrate thickness, with an intermediate dielectric layer interposed therebetween, this is, a broadside-coupled strip line.
In the case of adopting the broadside-coupled strip line, although the number of dielectric layers is inevitably increased, since the density of line in a state of being projected on the dielectric substrate main surface is equal to a single length of line, it is possible to achieve fine-pitch arrangement. Moreover, the opposed area between a pair of signal conductors can be increased. This makes it possible to render the electrical coupling between the signal conductors stronger than in the case of adopting the parallel microstrip line, and thereby improve the prospect of enhancement in cross talk characteristics (for example, refer to Japanese Unexamined Patent Publication JP-A 2005-51496).
Incidentally, in a packaging component or an electronic component, a pair of signal lines designed in internal layer pattern are led out to a substrate main surface via a through conductor. This makes it possible to establish electrical connection with an electrode terminal of a mounting circuit board.
However, in the case of adopting the broadside-coupled strip line, since the through conductors for use differ in height from each other, when a composite circuit board is constructed by mounting a packaging component or an electronic component on the mounting circuit board, considerable variations in line length will occur in the composite circuit board. This leads to an undesirable increase in the skew (propagation delay time difference) between differential signals that appears at the output end, which may result in degradation of transmission performance quality. Furthermore, as the signal speed is increased with consequent shortening of the wavelength of a propagating electromagnetic wave, the amount of phase shift is increased with respect to a single, common physical length. That is, the higher is the frequency level, the greater is the extent of skew.